Miscellaneous fixes for Lua subprocess() on Windows

0899594 bit me because of rg3/youtube-dl#4149. I killed mpv with Ctrl+C a couple of times, and I had two youtube-dl instances running in the background, each downloading the same video.

39040a3 adds two more support functions to the Windows code in lua.c. It's getting a bit big and I was wondering if it would be better to split it into multiple files (subprocess_win32.c and subprocess_posix.c?)